Automatic transmissions, especially for motor vehicles, encompass, in accordance with the state of the technology, have planetary gearsets which, with the aid of frictionally based shifting elements, such as clutches and brakes, can be engaged. Transmissions normally possesses a slip-action startup element and are optionally furnished with a bypass clutch. The bypass clutch would be supplied, for instance, in the form of a hydrodynamic torque converter or a hydrodynamic clutch.
A multi-speed transmission of this generic type is disclosed by DE 102 13 820 A1. This cited transmission includes one input shaft and one output shaft, which are mutually co-axially aligned. Further included is a total of three planetary gearsets and six frictional-based shifting elements. For the transfer of the speed of rotation of the input shaft to the output shaft, the cited transmission provides two power paths. Two of the planetary gearsets form a shiftable principal gearset which is designed as a two-carrier, four shaft transmission and is further known as a Ravigneaux planetary gearset or, alternately as a Simpson planetary gearset. The output element of the principal gearset is connected to the output shaft of the transmission. The remainder of the three planetary gearsets serve as simple planetary gearsets and form non-shiftable, input-sided gearsets, which are rigidly connected to the input shaft and generate on the output end, two speeds of rotation where, first, the speed of rotation of the input shaft, second, the torque can also be transmitted to various input elements of the principal gearset. By selective engagement of two of the six frictional shifting elements, which are namely clutches and brakes, a total of eight forward gearsets can be engaged without range shifting. In this way, when changing from one gearset into the next higher or the next lower gear, only one of the previously engaged shifting elements need be disengaged and one previously disengaged shifting element needs to be engaged.
Made known by U.S. Pat. No. 5,429,557, is an axially aligned, comparatively compactly assembled multi-speed automatic transmission with an input-sided gear group and an extended, principal planetary gearset constructed of two planetary gearsets, which are coupled together as in a two-carrier, four shaft planetary transmission. In an embodiment of this transmission, which serves well for installation in a vehicle having a front-end, transversely directed drive, the input shaft and the output shaft are arranged to be axis-parallel to one another. Starting with this arrangement, the input-sided gear group is constructed in the input-sided method for overdrive characteristics, with two spur gearsets of a constant ratio and the two planetary gearsets of the principal gearset are radially intermeshing, one within the other, wherein the axes thereof run parallel to the axis of the input shaft of the likewise transverse transmission. With a total of five frictionally based shifting elements, as recited in the known patent, only six forward, gears are operationally available without range shifting.
Furthermore, DE 199 49 507 A1 of the Applicant, discloses a multi-speed transmission with two non-shiftable, input-sided planetary gearsets placed on the input shaft, that generate two speeds of rotation which, together with the speed of rotation of the input shaft, can be transmitted to various input elements of a multi-speed principal planetary gearset by way of selective engagement of shifting elements acting upon the output shaft AB such that when changing from one gearset to the next higher or the next lower gearset, only one engaged shift element need be disengaged. The principal planetary gearset, as before, is once again constructed in the manner of a double planetary gearset as employed in a two-carrier, four shaft planetary gearset, wherein the two double planetary gearsets are rigidly coupled to one another. Such that when five shifting elements are used, seven forward gearsets can be implemented without range shifting. With the use of six shifting elements, nine or ten forward gearsets become available. All four planetary gearsets are, in this embodiment, co-axially aligned to each other and to the input shaft.
Within the framework of DE 101 15 983 A1 of the Applicant, a multi-speed transmission is described and comprises one drive shaft, which is connected with an input-sided gear group, and one output shaft which, in turn, is connected to an output sided gear group and further has a maximum of seven shifting elements, the selective engagement of which implements at least eight forward gearsets (without range shifting). The input-sided gear group is a shiftable or a non-shiftable planetary gearset or, alternately, a pair of non-shiftable, mutually coupled planetary gearsets. The output-sided planetary gear group is assembled as a two-carrier, four shaft transmission with two shiftable output-sided planetary gearsets and possesses four free shafts. The first free shaft of this two-carrier, four shaft transmission is connected with the first shifting element; the second free shaft is connected to the second and third shifting element; the third free shaft is attached to the fourth and fifth shifting element, and the fourth free shaft is connected to the output shaft. In a multi-speed transmission with a total of six shifting elements, it is proposed to additionally connect the third free shaft or the first free shaft of the output-sided gear group additionally to the sixth shifting element. For a multi-speed transmission having a total of seven shifting elements, it is proposed to also connect the third free shaft with the sixth shifting element and the first free shaft to a seventh shifting element. In these known embodiments, all planetary gearsets of the transmission are mutually co-axial.
A plurality of other multi-speed transmissions are disclosed by DE 101 15 995 A1 of the Applicant, wherein four shiftable, mutually coupled and co-axially arranged planetary gearsets and six or seven frictionally based shifting elements are provided the selective shifting of which transmits a speed of rotation of an input shaft of the transmission to an output shaft of the transmission, such that nine or eleven forward gearsets and at least one reverse gearset become available. According to the shifting diagram of the transmission, it may be seen that in each gearset two or three shifting elements are engaged while, upon a change from one gearset to the next successive higher or lower gearset, only one engaged shifting element need be disengaged and one previously engaged element need be engaged, while avoiding range shifting.
The present invention has the purpose of proposing a multi-speed transmission of the generic kind mentioned in the opening lines, which possesses at least eight forward gearsets and at least one reverse gearset and with the use of a total of four planetary gearsets, the least possible number of shifting elements need be included. In this arrangement, all four forward gearsets, when in a sequential gear change, can be so shifted without range shifting, in other words, in a case of change from one forward gearset to the successively next higher or lower forward gearset, respectively, only one previously engaged shifting element need be disengaged and a previously engaged shifting element need be disengaged. In addition, the transmission is expected to exhibit a large spread, where comparative harmonic gear separations are concerned and have a favorable degree of efficiency in the principal gearsets used mainly in driving. This means a comparatively small amount of slippage and tooth wear is incurred. The transmission should be adaptable, in particular, for usage with non co-axial input and output shafts and correspondingly, the axial length of the transmission should be held to the shortest possible dimensioning.